Conventionally, a pressure sensor is proposed as the above type of the semiconductor device (see, for example, the patent literature 1). Specifically, the pressure sensor includes a first substrate having one surface and an opposite surface. On the first substrate, a diaphragm portion is arranged by forming a concavity on the opposite surface, and further, multiple gauge resistors are formed on the diaphragm portion so as to provide a bridge circuit. More specifically, a well layer having a N conductive type is formed in the first substrate, and multiple gauge resistors are formed in the well layer. The second substrate (i.e., a cap substrate) is bonded to the one surface of the first substrate so as to form the air tight chamber between the first substrate and the second substrate and to seal the gauge resistors in the air tight chamber.
A diffusion wiring layer is formed in the first substrate by diffusing an impurity having a P conductive type in the well layer, and the diffusion wiring layer is appropriately and electrically connected to multiple gauge resistors. The diffusion wiring layer on the one surface of the first substrate is also bonded to the second substrate.
The above pressure sensor is used for detecting pressure of an oil discharged from an oil pump, for example. When a measurement medium is introduced into the concavity formed in the first substrate, the diaphragm is deformed in accordance with a pressure difference between the pressure of the measurement medium and the pressure in the air tight chamber (as a reference pressure chamber). Accordingly, the gauge resistor formed on the diaphragm is also deformed, so that the output voltage of the bridge circuit is varied, and the sensor signal according to the pressure difference is output.
However, as described above, when the device includes the diffusion wiring layer, a micro protrusion is formed since an impurity concentration in the diffusion wiring layer is different from an impurity concentration in a portion in which the diffusion wiring layer is not formed. Since the micro protrusion is formed by the difference of the impurity concentration, the protrusion is formed along a boundary between the diffusion wiring layer and the portion in which the diffusion wiring layer is not formed. Accordingly, when the first substrate is bonded to the second substrate, a space (i.e., a loose bonding portion) attributed to the protrusion is formed between the first substrate and the second substrate.
Then, for example, as shown in regions A to D in FIG. 8, the diffusion wiring layer J19a to J19d may reach an edge of the one surface of the first substrate. Specifically, the boundary between the diffusion wiring layer J19a to J19d and a region in which the diffusion wiring layer J19a to J19d is not formed is disposed at the edge of the one surface of the first substrate. Here, a dotted line in FIG. 8 shows the boundary between a portion arranged in the air tight chamber and a portion connected to the second substrate. In FIG. 8, the diffusion wiring layers J19a to J19d are hatched to easily understand the drawing although the drawing is not a cross sectional view.
In the above case, as described above, the protrusion is formed between the diffusion wiring layer and the region in which the diffusion wiring layer is not formed. When the space attributed to the protrusion is formed between the first substrate and the second substrate, the space is communicated with an outside air. Accordingly, the air tight chamber is communicated with the outside air via the space attributed to the protrusion, and therefore, leakage of the air tight chamber may occur.